1. Field of the Invention
The present invention relates to a scheme for automatic adjustment of electron optics system, and for astigmatism correction in a charged particle beam optical system, of an electron optics device such as a scanning electron microscope.
2. Description of the Background Art
The Scanning Electron Microscope (SEM) is widely used in a variety of fields including the semiconductor inspection. In order to inspect the fine structure on a surface of a sample such as a semicondutor at high magnification, it is necessary to have the electron optics system of the SEM accurately adjusted all the time. In other words, a clear image of the fine structure on the sample cannot be obtained unless the SEM electron optics system is adjusted in such a manner that the electron beam to be irradiated from an electron gun of the SEM is sufficiently narrowed down by the electron optics system so that the electron beam is passing through a center of the electron optics system and focused on a single point on the sample.
There are several types of the adjustment operations for such an electron optics system, and among them, an adjustment operation for aligning an objective lens aperture stop so that the electron beam passes through a central axis of the electron optics system is called the objective lens alignment.
If this objective lens alignment is inaccurate, that is, if a center of the objective lens aperture stop is not aligned with the axis of the electron beam, a sample image moves within the image field when an operation to adjust the focal point with respect to the sample is carried out. This situation can cause the so called astigmatism in which a shape of the irradiated electron beam spot on the sample deviates from a true circle, and lead to the lowering of the image resolution. For this reason, usually, the operator manually operates the objective lens while outputting an over-focused image and a under-focused image alternately, and adjusts a position of the objective lens aperture stop to a position at which an object image no longer moves within the image field.
The objective lens alignment is easily affected by the external vibrations, etc. exerted on the SEM, so that there is a need to adjust the objective lens alignment either regularly or whenever the SEM is used, and this adjustment operation is usually realized by the manual operation of the operator which is carried out while watching the SEM image. However, this adjustment operation is a kind of operation that requires skills and there are not many operators who can carry out this adjustment operation accurately, so that it is difficult for a general SEM user to carry out this adjustment operation accurately.
In addition, when the SEM is used for the semiconductor inspection, for example, a time required for the judgement of the electron optics system largely affects the throughput of the inspection, and this has conventionally been a factor for lowering the throughput of the semiconductor inspection process, so that there is a demand for a mechanism to automatically realize the adjustment of the electron optics system in the SEM.
In further detail, the SEM has been used for the purpose of observing the surface state of the semiconductor element in recent years, and its resolution has been as good as 1 nm or less. In the following, a general configuration of this type of SEM will be briefly described
In the SEM of FIG. 1, a field emission type electron gun 3 is formed by a cathode 1 and anode 2, and a high luminance electron beam 4 is emitted from this electron gun 3. The electron beam 4 is narrowed through a condenser lens 5, an aperture stop 7 and an objective lens 6, and irradiated onto a surface of a sample 8. At this point, the secondary electrons emitted from the sample surface are detected by a detector 11. The electron beam 4 is two-dimensionally scanned over the sample 8 mounted on a sample holder 9 by means of a deflector 10 which is controlled by a scanning control circuit 12. The information on the sample surface is obtained by displaying signals from the detector 11 on an image display device 13 in synchronization with this scanning operation.
Now, in this type of focusing system using the objective lens 6, there is an aberration called astigmatism in general. This astigmatism is caused because the focused position of the electron beam is displaced along two axial directions perpendicular to the beam axis. When this type of aberration exists, a beam cross section shape on the sample surface changes as shown in parts (a), (b) and (c) of FIG. 2 depending on the focal lengths of the objective lens 6. Here, a beam radius in a state where a circular beam as shown in a part (b) of FIG. 2 is obtained becomes larger than that in a case where no astigmatism exists. This aberration is caused because the focal lengths in two directions are different.
For this reason, conventionally, a mechanism 15 for generating the quadruple electric or magnetic field called "stigmater" is provided inside a charged particle beam optical system formed by the electron gun 3, the condenser lens 5, the objective lens 6 and the aperture stop 7, so as to correct the astigmatism. More specifically, the quadruple stigmater formed by four electromagnets or electrodes is provided in a state capable of being rotated for 45 degrees around the optical axis below the objective lens 6, or two sets of quadruple stigmaters are provided at relative angle of 45 degrees with each other and their intensities are adjusted, so that the focused positions on two directions, X direction and Y direction, coincide with each other.
Next, with reference to FIG. 3, the conventionally used astigmatism correction scheme will be described. In a case of observing a sample as shown in a part (a) of FIG. 3, when the astigmatism exists, that is, when the beam cross section shape as shown in a part (a) of FIG. 2 is used for example, the image displayed on the display device 13 appears to have a direction in which the edge resolution is high and a direction in which the edge resolution is low as shown in parts (b) and (c) of FIG. 3, respectively.
Then, when the focal length of the objective lens 6 is changed so that the beam cross section shape becomes as shown in a part (c) of FIG. 2, a direction with a high edge resolution and a direction with a low edge resolution are switched. In such a case, the stigmater 15 is adjusted so that the directionality of the image disappears even when the focal length of the objective lens 6 is changed. At this point, the beam cross section shape changes as shown in parts (a), (b) and (c) of FIG. 4 where only the beam radius of the circular beam is changed.
In a case of carrying out this astigmatism correction automatically, the following operations will be required. First, contrasts of images obtained by using different lens focal lengths are obtained and a position at which the contrast is maximum is determined. When the astigmatism is large, there are cases in which two local maxima appear, and in such a case, a central point between two local maxima is determined. Then, at the focal position determined in this manner, the contrast is maximized by adjusting the stigmater in the X direction. Next, the contrast is maximized by adjusting the stigmater in the Y direction. By repeating the above operations, a converging position is determined as the optimum position.
However, this type of astigmatism correction scheme has been associated with the following problems. Namely, the stigmater has been adjusted by an operator while watching the image displayed on the display device, so that the adjustment of the stigmater has been relying on a visual sense of the operator and therefore an accuracy of the adjustment depends on skills of the operator. In addition, if the adjustment of the stigmater is not perfect, the observation precision is lowered. Moreover, it has been time consuming as it has been done manually, and therefore it has been required to irradiate beams onto the sample more than absolutely necessary. This in turn causes a damage on the sample or a change in the resist shape, so that it is generally undesirable. Furthermore, when the sample has a directionality, the correction is difficult.
Also, the conventional scheme for automatic astigmatism correction has only a limited range that can possibly be corrected compared with the manual correction in a case of dealing with a practical sample, and its accuracy is inferior to that obtained by the manual correction in most cases.
In addition, in a case of observing the sample as shown in a part (a) of FIG. 3, when the beam cross section shapes are as shown in parts (a), (b) and (c) of FIG. 5 for example, the resolutions are nearly equal in two directions perpendicular to two sides of the sample, so that the astigmatism correction is difficult. This situation is the same in a case of automatic astigmatism correction.
Note that the above noted problems are not only relevant to an observation device such as SEM, but also relevant to a charged particle beam drawing device using electron beam or ion beam as well.